Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Abstract

We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye Ru II (2,2′-bipyridyl-4,4′-dicarboxylate) 2 (NCS) 2 (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO 2 ) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO 2 films were compared with those obtained for control dye-coated ZrO 2 films, as the high conduction band edge of ZrO 2 prevents electron injection. Adsorption of the dye onto the TiO 2 film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data indicated that electron injection in these films occurs in e10 -12 s. Detailed analysis indicates the injection is at least biphasic, with ∼50% occurring in <150 fs (instrument response limited) and 50% in 1.2 ( 0.2 ps. These ultrafast electron injection kinetics are contrasted with the charge recombination reaction, which occurs on the microsecond-millisecond time scales. The ultrafast rate of electron injection observed here is critical both for the high energy conversion efficiencies obtained with this sensitizer dye, and for the excellent long-term stability of this dye in photoelectrochemical solar cells.

Nanocrystalline Mesoporous Strontium Titanate as Photoelectrode Material for Photosensitized Solar Devices: Increasing Photovoltage through Flatband Potential Engineering

“…Charge injection in zirconia is not thermodynamically feasible, and, as a result, the ground-state absorption of the dye is promptly recovered by deactivation of the excited state (eq 3), whose lifetime on the oxide surface is estimated to be τ 1/2 = 15 ns. 18,19 From the very long lifetime of the bleaching signal observed for the dye adsorbed on SrTiO 3 , it is thus concluded that charge injection (eq 2) does indeed occur quantitatively (Φ inj > 0.8) into this material. The slow recovery of the initial absorbance of the adsorbed dye is attributed to the back electron transfer reaction (eq 5).…”

pH-Dependent Electron Transfer from Re-bipyridyl Complexes to Metal Oxide Nanocrystalline Thin Films

“…Under all pH conditions, negligible oxidized peak amplitude was observed at <200 fs, indicating a lack of instantaneous injection component that was observed for RuN3 and derivatives on TiO 2 . 36,[63][64][65][66][67][68][69] This has been attributed to the insertion of a CH 2 spacer between the bipyridine and the anchoring group, which reduces its electronic coupling with TiO 2 , slowing down the electron injection rate. 40 At pH 0, the excited-state CO stretch band disappears completely by ∼50 ps, with a concurrent appearance of the oxidized-state band.…”

Injection Kinetics and Electronic Structure at the N719/TiO2 Interface Studied by Means of Ultrafast XUV Photoemission Spectroscopy

“…[7][8][9][10][11] This feature is associated with the population dynamics of different metalto-ligand charge-transfer (MLCT) states of the dye complex. Following excitation at the maximum of the absorption band (typically at a wavelength of 530 nm), the dye is initially prepared in the singlet 1 MLCT state.…”